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Cordierite Ceramics

Cordierite is a man-made refractory low thermal expansion crystalline solid that forms at very high temperatures (in the right mix of kaolin and talc).

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Details

Cordierite ceramics are well known for their low thermal expansion and refractory character. Although cordierite is available as a powder, here we are generally talking about ceramic that went into the kiln as an ordinary composite of mineral powders but emerges as a solid cordierite crystalline matrix (grown by the firing process tuned to its needs).

Cordierite makes the ceramic manufacture of products like catalytic converters possible. Barrett’s Minerals, near Dillon, MT, claims to make the talc used in making the converters for all cars in North America. They constantly heat up and cool down quickly and must not crack because of the shock. These converters get their thermal shock resistance properties from a bonded matrix of low-expansion cordierite crystals.

Cordierite crystals form during firing between 1200-1400C (~cone 6-15). Interestingly, cordierite crystals have a lesser thermal expansion along one axis than another. The ideal chemistry to produce the crystals is 13.8% MgO, 34.8% Al₂O₃ and 51.4% SiO₂ (2MgO, 2Al₂O₃, 5SiO₂). Talc, kaolin and raw alumina powder can be blended to source this chemistry (other materials are also used e.g. aluminum hydroxide, steatite and other MgO minerals). Multiple factors determine the temperature and extent to which cordierite develops. Materials sourcing more than one oxide (like kaolin and talc) may react better than pure oxide materials (like MgO, Al₂O₃, SiO₂). Materials of finer particle size react better. Better cordierite development occurs when the precursor mix is pressed to higher densities and firing schedules are tuned (in rate-of-climb, ultimate temperature and holding patterns). Thermal expansion measuring equipment is needed to study the relationships between materials, process and firing on the quality of cordierite produced. As noted, the ultimate goal is the development of a continuous crystal matrix of cordierite (not just the presence of the crystals in an otherwise typical ceramic).

Lacking testing equipment a potter can produce cordierite to some extent, and therefore make ware having greater resistance to thermal shock failure. A simple firing of a body having the above recipe to cone 10 or 11 and held should produce some crystal development. There is also a more direct approach: Cordierite powder is available, it is prefired and finely ground. It can be plasticized for forming and bonded during firing (at much lower than normal cordierite creation temperatures) to produce a ceramic with lower expansion.

Cordierite is very practical as a refractory for kilns shelves and kilns posts because of its combination of low cost and low thermal expansion. Of course, it is not as refractory as alumina or SiC so is generally suitable for light duty use.

While cordierite ceramic vessels could be made (e.g. ovenware), the material has such a low thermal expansion it is very difficult (or impossible for most technicians) to match a glaze (without crazing). Notwithstanding this, many recipes can be found for cordierite ovenware or flameware. However, these are seldom fired above cone 10 so any thermal shock resisting behavior they have is attributable to grog content or their open fired matrix rather than to any cordierite development.

Related Information

An electric kiln half shelf that costs $500!

This picture has its own page with more detail, click here to see it.

This Advancer Nitride-bonded Silicon Carbide shelf is 26 inches wide (by 1/4 inch thick) weighs 9 lbs. These are incredible durable and strong. However there are cautions to their use. They can act as an electrical conductor so must not contact elements and should not be used in kilns with unpinned elements protruding from grooves. They must be stored in a dry place to prevent moisture penetration (which can cause a steam explosion during heatup). The company has a recommend drying schedule if shelves do absorb moisture (the application of kiln wash is not considered a prolonged exposure and is OK).

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